A tool holder normally includes a tool, a tapered portion and a pull stud bolt. When the tool holder is attached to a machine spindle, the pull stud bolt is drawn inwardly by way of a collet chuck in preloading. The tapered portion of the conventional tool holder is defined by a single continuous smooth tapered surface. An opening for receiving the taper portion of the tool holder is formed on the machine spindle. The opening in the machine spindle is defined by a single continuous smooth tapered surface. The tool holder is attached to the machine spindle in such a manner that the tapered portion of the tool holder is engaged with the corresponding tapered surface of the reception opening of the spindle over its entire length.
Nowadays a workpiece to be machined is often made of aluminum, non-ferrous metal containing resins, hard alloys and so on. These materials require high speed cutting or very high speed cutting. Even when a normal metal workpiece is machined, high speed cutting is desirable for accurate machining.
When the tool holder attached to the machine spindle is driven at high rate of rotation, the outer (entry) area of the tapered reception opening of the spindle is enlarged, e.g. by few micron meters. As a result, only the smaller diameter (end) area of the tapered support portion of the tool holder is engaged with the interior surface of the spindle, therefore play of the tool sometimes occurs during machining. Such play in the tool is allowed by the small clearance formed at the large diameter area (end) of the spindle opening. Because the small diameter area is hardly enlarged, the tool holder is supported by only the small diameter end of the support portion. It is hard to machine a workpiece with high accuracy if such tool play occurs. The play will also shorten the tool life. Moreover, severe cutting conditions must be avoided because of the play, to the detriment of productivity.